1. Field of the Invention
The present invention relates to a continuously variable transmission system for a vehicle using a combination of a continuously variable transmission and a planetary gear train.
2. Description of the Related Art
Such a continuously variable transmission system for a vehicle is disclosed in Japanese Patent Application Laid-open No. 10-196759.
According to the known continuously variable transmission system, when the vehicle travels at a low speed, the engine torque is transmitted to the driven wheels only by a toroidal type continuously variable transmission. When the vehicle is traveling at a high speed, the engine torque is transmitted to the driven wheels by both the toroidal type continuously variable transmission and a planetary gear train, thereby achieving a wide range of ratios that cannot be achieved simply by using the toroidal type continuously variable transmission.
In order to control the ratio of the toroidal type continuously variable transmission, an electronic control device, such as a solenoid valve, is used. However, if the electronic control system fails, the ratio of the toroidal type continuously variable transmission cannot be controlled in some cases. If an electronic control device failure occurs when the vehicle is stationary, when the vehicle starts a load that changes the ratio of the toroidal type continuously variable transmission beyond the LOW ratio or a load that changes the ratio thereof beyond the OD ratio is generated, a large load is applied to either the input discs or output discs of the toroidal type continuously variable transmission, resulting in the possibility that the durability could be degraded. Furthermore, if the ratio of the transmission is fixed at the OD ratio, a torque transmitted to the driven wheels decreases, resulting in the possibility that the starting performance could substantially degrade or the vehicle speed could become excessive due to a high ratio when the vehicle reverses.
It is an object of this invention to overcome the above-described drawbacks.
More specifically, it is an object of this invention to prevent degradation of the starting performance and the durability of the continuously variable transmission of a continuously variable transmission system for a vehicle by controlling the ratio of the continuously variable transmission even when the electronic control system of the continuously variable transmission fails.
In order to achieve the above-mentioned object, in accordance with a characteristic of the present invention, the continuously variable transmission system of this invention includes a continuously variable transmission having an input member into which a torque of an engine is input and an output member to change the speed of rotation of the input member and to output the rotation. A planetary gear train has first, second, and third elements, wherein the first element is connected to the output member of the continuously variable transmission. The second element is connected to driven wheels. A first clutch engages the first element of the planetary gear train with the second element thereof. A second clutch having an input side connected to the engine and a shifter that connects the output side of the second clutch and the third element of the planetary gear train when the vehicle travels forward and connects the output side of the second clutch and the driven wheels when the vehicle travels in reverse.
In accordance with the above-mentioned arrangement, when the vehicle travels forward under normal conditions, that is, when there is no failure of the electronic control system, the output side of the second clutch is connected to the third element of the planetary gear train by the shifter and the first clutch is gradually engaged while the engagement of the second clutch is canceled. The torque of the engine is then transmitted to the drive wheels via the continuously variable transmission and the first clutch, thereby starting the vehicle. When the first clutch is completely engaged as the vehicle speed is increasing, the vehicle accelerates as the continuously variable transmission changes from a LOW ration to an OD ratio, that is a direct mode. When the ratio of the continuously variable transmission reaches the OD ratio, the second clutch is engaged and the engagement of the first clutch is canceled. The torque of the engine is input into the third element of the planetary gear train via the second clutch. A majority of the torque is then transmitted from the second element to the driven wheels to accelerate the vehicle. A proportion of the torque is then returned back to the engine side from the first element via the output member and the input member of the continuously variable transmission, that is, a torque slip mode. When the continuously variable transmission is shifted from the OD ratio to the LOW ratio in this state, the ratio of the entire continuously variable transmission system changes beyond the OD ratio of the continuously variable transmission to accelerate the vehicle.
When the vehicle travels in reverse under normal conditions, that is, when there is no failure of the electronic control system, the output side of the second clutch is connected to the driven wheels by the shifter and the clutch is gradually engaged. The torque of the engine is then transmitted to the driven wheels via the second clutch and the shifter, starting the vehicle. After the vehicle has started, the second clutch is completely engaged and the vehicle is accelerated by increasing the rotational rate of the engine.
When the vehicle travels forward at a time when the electronic control system of the continuously variable transmission has failed, the vehicle is started by connecting the output side of the second clutch to the third element of the planetary gear train by means of the shifter and gradually engaging the first clutch and the second clutch. In this case, the ratio of the continuously variable transmission is set at a predetermined ratio between the LOW ratio and the OD ratio so the engagement forces of the two clutches balance each other. When the first clutch is completely engaged, the ratio of the continuously variable transmission changes from the predetermined ratio toward the OD ratio. Subsequently, when the second clutch is completely engaged, the rotational rate of the engine is increased while the ratio of the continuously variable transmission is fixed at the OD ratio, and the vehicle thus accelerates. During this period, the torque of the engine is transmitted to the driven wheels via the second clutch, the shifter and the planetary gear train. Therefore, the continuously variable transmission only carries out the speed change and is not involved in the transmission of torque. By engaging the first and second clutches with predetermined engagement forces when the vehicle travels forward at a time when the electrical control system has failed, it is possible to prevent the ratio of the continuously variable transmission from changing beyond the LOW ratio or the OD ratio. Accordingly, generation of an excessive load can be prevented, thereby enhancing the durability of the continuously variable transmission.
When the vehicle travels in reverse when there is a failure of the electronic control system, the output side of the second clutch is connected to the driven wheels by the shifter and the second clutch is gradually engaged. The torque of the engine is thus transmitted to the driven wheels via the second clutch and the shifter, thus starting the vehicle. After the vehicle has started, the second clutch is completely engaged, and the rotational rate of the engine is increased so as to accelerate the vehicle. In this case, since the torque of the engine does not pass through the continuously variable transmission, it is possible to prevent degradation of the durability of the continuously variable transmission due to an excessive load.
In the cases of starting to travel in both the forward and reverse directions, since the ratio of the continuously variable transmission is not fixed at the OD ratio, the torque to be transmitted to the driven wheels does not decrease, which would greatly degrade the starting performance, and the vehicle speed does not become too high due to a high ratio after the vehicle has started to reverse.